1. Field of the Invention
The present invention relates to aqueous dispersions with reduced triethylamine content, a process for the preparation of such dispersions, and their use as binders for coatings.
2. Description of the Prior Art
With the aim of reducing emissions of organic solvents, aqueous coating compositions are increasingly replacing solvent-containing compositions. These aqueous compositions contain polymers, which are similar to those used in solvent-containing coatings with the exception that they contain hydrophilic groups to render the polymers water soluble and/or water dispersible. It is advantageous if these hydrophilic groups are incorporated into the polymer ("internal emulsifiers"), instead of being physically mixed with the polymer ("external emulsifiers").
The internal hydrophilic groups are anionic groups usually formed, e.g., from a carboxylic acid and a base. It is known from U.S. Pat. No. 3,412,054 to hydrophilically modify polyurethanes by incorporating compounds containing hydroxy and carboxylic acid groups, such as dimethylolpropionic acid, and neutralizing the
Bases (for example, metal salts such as alkali metal hydroxides or amines) are used to neutralize the incorporated hydrophilic groups. Amines have an advantage over metal salts because they evaporate and, thus, do not remain in the dried paint film after application of the binder. Therefore, the hydrophilicity and water sensitivity of the coating is significantly reduced. However, primary and secondary amines have the disadvantage that they react with isocyanates.
Isocyanate group-containing compounds are used for preparing aqueous dispersions. Particularly in the preparation of polyurethane dispersions (see for example D. Dieterich, Prog. Org. Coatings 9, 281 (1981), EP-A 220,000, EP-A 511,547 and WO 98/05696), prepolymers are often obtained in intermediate stages that contain free isocyanate groups as well as neutralizing bases.
Also, in the application of aqueous two-component polyurethane coating compositions containing polyisocyanate crosslinking agents and aqueous polyol dispersions (see for example EP-A 358,979, EP-A 537,568 and EP-A 542,105) neutralizing agents and isocyanate groups from the crosslinking agent come into contact with one another.
Particularly in these two cases, tertiary amines free of hydroxyl groups are increasingly used as neutralizing agents since they do not react with isocyanate groups. Due to its ready availability, triethylamine is generally used as the neutralizing amine. However, the release of corrosive triethylamine from aqueous coating compositions is a disadvantage for ecological reasons. Accordingly, paint users as well as paint manufacturers have attempted to reduce the triethylamine content in aqueous dispersions with out impairing paint quality.
Replacement of triethylamine with ammonia, primary and secondary amines, hydroxyl group-containing tertiary amines and tertiary amines having relatively long alkyl substituents alternatives have not been successful.
Ammonia reacts rapidly with isocyanate groups. In addition, ammonia-neutralized binders in pigmented paints exhibit considerably worse pigment wetting.
Primary and secondary amines, as well as hydroxyl group-containing tertiary amines, react with isocyanates. Hydroxyl group-containing tertiary amines are also considerably less volatile than triethylamine, which is reflected in the poorer water resistance of the coating in the first few days after paint application.
Tertiary amines with relatively long alkyl substituents also have the disadvantage compared to triethylamine of insufficient volatility. Experience has also shown that resins neutralized with long chain tertiary amines are more difficult to disperse than triethylamine-neutralized resins.
An object of the present invention is to provide an amine for neutralizing aqueous dispersions that is at least an equivalent replacement for the ecologically less desirable triethylamine, but which does not have the disadvantages of the previously discussed alternative neutralizing amines for aqueous dispersions.
This object may be achieved according to the present invention by using N,N-diisopropyl-N-ethyalmine as the neutralizing amine in the preparation of aqueous dispersions.